1. Heat oven to 350°F. Line an 8-inch square baking pan with foil, letting ends of foil hang over side of pan. Lay phyllo on a flat surface and brush with about 2 teaspoons of the butter. Fold phyllo in half, buttered side in, and place in pan. Fold sides of phyllo down to fit tightly into the bottom of the pan.
2. Combine apple, pear, and 1/4 teaspoon of the cinnamon in a bowl. Transfer to pan, spreading evenly over phyllo.
3. Mix oats, walnuts, pecans, sugar substitute, and remaining 1/4 teaspoon cinnamon in a bowl. Add remaining butter and stir to combine. Crumble over fruit. Bake until topping is crunchy and fruit has softened, 25 to 30 minutes. Cool slightly; use foil to lift tart out of pan to make cutting easier.